1. Field of the Invention
The present invention relates in general to digital signal processing and in particular to circuits, systems and methods for digital to analog digital conversion.
2. Description of the Related Art
To date, most audio formats have used PCM encoding or an encoding that is decoded into a PCM format. As an example, a compact disk is recorded with a 16-bit format, and a 44100 Hz sample rate. Newer audio formats use up to 24 bits, and up to 192 kHz sample rates. In these cases the DAC receives PCM data and passes the data through an interpolation filter to increase the sample rate. A delta-sigma modulator then reduces the number of bits representing each sample, for example from 24-bit samples to 1-bit samples (this is called a single bit modulator) or to 4-bit samples (a multi-bit modulator). Modulator performance is typically specified in terms of its Modulation Index or MI, which is the ratio of the maximum allowable signal peak modulator input to the mathematical maximal modulator input (equivalent to the signal peak of the feedback signal). For example, if the MI is 0.5 and the modulator has a single-bit bipolar output range, the maximum allowable input produces an output that is 75%+1 and 25%xe2x88x921 for an average of 0.5. The delta-sigma modulator creates significant quantization noise; however, the delta-sigma modulator has the ability to shift this self-generated noise out of the signal band.
If a multi bit modulator is used, the 4-bit data from the delta-sigma modulator is next thermometer encoded to represent 16 levels. The thermometer encoded data is passed through dynamic element matching logic implementing an algorithm for shaping the noise to account for digital to analog converter (DAC) element mismatch.
The DAC, which ultimately converts the digital data to analog for eventual presentation to the listener as audio, is often a switched-capacitor circuit that also provides filtering, although continuous time circuits may be more appropriate.
One advantage of multi-bit systems is that a higher modulation index can be used, meaning that the output signals can be of a greater level. The greater signal level directly improves the signal to noise level.
The newer Sony/Philips 1-bit recording system (xe2x80x9cSuper Audio CDxe2x80x9d or xe2x80x9cSADCxe2x80x9d) stores data from an analog modulator onto the given digital storage media in a 1-bit format. As a result, techniques for converting data in the 1-bit digital format to analog must be developed. This is a non-trivial problem since such factors as filtering out of band noise, gain control through the modulator, and hardware minimization must be considered. Moreover, it is usually a requirement that a dynamic range of xe2x88x92120 dB in the audio band be achieved.
The conversion of one bit data normally does not gain the level advantages that the multi-bit modulator has. One such technique is disclosed in co-pending and co-assigned U.S. Pat. No. 6,011,501 entitled xe2x80x9cCIRCUITS, SYSTEMS AND METHODS FOR PROCESSING DATA IN A ONE-BIT FORMATxe2x80x9d by Gong, et al. Granted Jan. 4, 2000.
Given the potential for wide acceptance of the Sony/Philips 1-bit audio format, and the continuous demand for improved sound quality, circuits, systems and methods for digital to analog conversion of 1-bit audio data are required.
According to one embodiment the principles of the present invention, a digital to analog converter is disclosed which includes re-coding circuitry with a is gain of greater than one for re-coding a received stream of 1-bit digital data into a stream of multiple-bit data. A multiple-bit digital to analog converter converts the stream of multiple-bit data into analog form.
The advantageous use of a multiple-bit DAC to perform the digital to analog conversion takes advantage of the higher modulation indices typically found with multiple-bit DACs vis-a-vis single bit DACs. Additionally, droop at the signal band edges can be substantially reduced or eliminated. In other words, a 1-bit DAC can be build and operated which has the high MI and constant gain of a multi-bit system. Further, these principles are flexible and can implemented in any one of a number of ways, including through the use of a gain stage in front of the multiple-bit DAC which is either a multiplier or a filter, such as a finite impulse response filter.